1. Field of the Invention
The present invention relates to a method and apparatus for the dispensing of an adhesive material susceptible to microwave heating. More particularly the present invention is concerned with varying the dispensing temperature of the material throughout the dispensing cycle such that the material is heated to different temperatures at different locations on a component.
2. Description of the Related Art
Microwave heating offers numerous advantages over more traditional heating methods. Microwave heating provides low thermal capacitance, volumetric heating, high energy transfer efficiencies, and practically instantaneous control of dispensing temperature. It is therefore generally known to advantageously use the properties of microwave energy for the curing and preheating of various resin systems.
European patent number 0618057B1, which is commonly assigned and is hereby incorporated by reference, discloses a resin transfer molding method in which resin is heated by microwave energy immediately before the resin is introduced into a mold cavity. The European patent teaches an apparatus and method for heating a resin passing through a resonator chamber. The invention was useful for reducing the resin viscosity and the resin transfer molding cycle time. The resin in this system was not dispensed along different locations of a component. It was injected through a fixed resin inlet port to form a molded part.
U.S. Pat. No. 5,064,494, teaches a method of applying an adhesive to a body panel by pre-heating and at least partially curing the adhesive by microwave energy. The adhesive is fed through a tube and the tube was placed within a microwave cavity. Microwave energy is directed into the cavity to heat the adhesive within the tube. The U.S. Pat. No. 5,064,494 ('494) patent teaches uniformly heating all of the adhesive within the tube. The adhesives commonly used for bonding automotive panels such as those described in the reference are highly viscous. The adhesive moves much faster through the central portion of the tube than at the radial surface. Consequently, adhesive material at the inner surface of the tube is resident much longer in the chamber than adhesive passing through the center of the tube. Under the condition described in '494, the adhesive at the radial surface would be heated to a higher temperature than adhesive passing through the center of the tube due to its longer residence time. For reactive adhesives, this would cause the material at the radial surface to react more than material at the center of the tube. The reacted adhesive would tend to constrict the diameter of the tube, and would eventually plug the tube completely.
The '494 patent also teaches directing microwave energy along a length of the component after the adhesive has been dispensed along an elongated bond path. Discrete locations are heated to promote adhesive curing. This reference does not teach heating the adhesive to different temperatures along the bond path. Furthermore, this patent teaches heating the adhesive passing through the chamber to one temperature and dispensing the adhesive along a path at approximately an equal temperature. When the dispensing path is elongated (i.e. greater than 2 meters), the adhesive dispensed early in the dispensing cycle cools more than the adhesive dispensed later in the cycle due to the substrate acting as a heat sink. This uneven temperature affects the bonding properties of the adhesive. It is desirable that the dispensing unit control the temperature of the material such that temperature variations from the first dispensed material and the latter dispensed material be reduced when the adhesive is placed between the members to be joined. It is also desirable that the microwave heating device not overly heat material near the radial surfaces of the dispensing tube. Furthermore, it is desirable to finely control the dispensing temperature of a material while the material is dispensed along an elongated bond path. The temperature of the material may be profiled such that different locations along the bond path receive material at different temperatures.
These and other desired objects of the instant invention will become more apparent in the course of the following detailed description and appended claims. The invention may best be understood with reference to the accompanying drawings wherein illustrative embodiments are shown.